This is a complicatied system, but there are a few things you can
check.  The most likely cause for a leak would be a hose or the
charcoal canister its self. Always check the hoses first, a
disconnected or damaged hose is most likely to cause the malfunction.
There should be a vacuum hose routing diagram on the underside of the
hood.  Determine if all hoses are routed correctly and replace any
damaged hoses.  If everything looks good, there are some more
complicated tests that can be done, i dont remember them off hand but a
haynes manual will help if you plan on doing the work yourself.
Otherwise a mechanic can do them for you.
Hope this helps.
P.S. make sure you tighten the gas cap at least 3 clicks or the check
engine light may come on.